The present invention relates to improved reflective articles, including improved diffuse reflective articles formed from a thermoplastic polymer and a diluent employing thermally induced phase separation technology (TIPS).
Diffuse reflection provides reflective light luminance at many angles, in contrast to specular or mirror reflection in which light is reflected back only at an angle equal to that of the incident radiation. Typical diffuse reflectors, used for example as white standards for various light measuring test instruments, are made of white inorganic compounds (such as barium sulfate or magnesium oxide) in the form of pressed cake or ceramic tile, all of which are expensive, stiff, and brittle. Other existing diffuse reflectors include (1) microvoided particle-filled articles that depend on a difference in index of refraction of the particles, the surrounding matrix and optional air-filled voids created from stretching and (2) microporous materials made from a sintered polytetrafluoroethylene suspension.
Another useful technology for producing microporous films is thermally induced phase separation (TIPS). TIPS technology has been employed in the preparation of microporous materials wherein thermoplastic polymers and a diluent are separated by a liquid-liquid phase separation as described in U.S. Pat. Nos. 4,247,498 and 4,867,881. A solid-liquid phase separation process has been described in U.S. Pat. No. 4,539,256. The use of nucleating agents incorporated in the microporous material is also described as an improvement in the solid-liquid phase separation method, U.S. Pat. No. 4,726,989.
Although existing TIPS microporous films are useful, effective but inexpensive diffuse reflective articles are still needed for the many diverse light management applications that are being developed. Many such applications require that diffuse reflective articles be as thin as possible, particularly when the diffuse reflective articles are used in electronic displays, such as liquid crystal displays (LCD""s) incorporated into notebook computers, handheld computers, portable phones, and other electronic devices. An additional attribute useful in diffuse reflective articles is controlled or reduced shrinking of the reflective articles over time and upon exposure to heat. Many polymeric materials, including those used in various TIPS microporous films, undergo noticeable shrinkage over time, particularly when exposed to heat. Reduction or elimination of this shrinkage is desired in order to produce an optimal diffuse reflective article. Furthermore, improved reflective articles are necessary that efficiently reflect light evenly and efficiently.
It is known from optical physics that incident light or radiation is significantly scattered and diffusely reflected by an article that is made from two or more non-absorbing materials having different refractive indices if the structure of the article provides appropriately sized regions of each material. To make an efficient diffuse reflector, the size (i.e. the cross-sectional width or height) of the light-scattering regions or sites of the article should be about the same as the wavelength of the light to be reflected. If the sizes of the scattering sites are a great deal smaller than the wavelength of interest, the light passes through the article. If the sizes are a great deal larger, the overall thickness required to diffusely reflect most of the light is prohibitively large. It is also known that the efficiency of the reflector is increased as the difference in refractive index of the two media is increased.
The present invention provides a diffuse reflective article incorporating microporous layers using TIPS technology. The diffuse reflective article is preferably flexible and can diffusely reflect radiation, e.g., visible light having a wavelength of from 380-730 nanometers (nm), more efficiently than most other known reflectors of similar thickness. The diffuse reflective article can provide improved reflectivity with raw materials that are both inexpensive and readily available. The measured reflectivity of the diffuse reflective article is dependent on the test method used, as many tests are inaccurate in measuring absolute reflectance values approaching 100%. In addition, extremely high diffuse reflectivity of these TIPS articles has been found in the near infrared and ultraviolet wavelengths.
The present invention further provides an improved diffuse reflector having a reduced thickness while maintaining a high absolute reflectance value. This reduced thickness allows for creation of various products having a narrowed profile, including LCD illumination systems.
In specific implementations, the diffuse reflector also has improved dimensional stability relative to prior TIPS articles. This improved dimensional stability allows for the diffuse reflector to be incorporated into products in a manner not possible with existing TIPS reflectors. For example, the reduced shrinkage allows incorporation into LCD illumination systems having a narrow perimeter bezel to hold the diffuse TIPS article in place.
In certain embodiments of the invention, the diffuse reflective article includes surface structures that reduce optical coupling (also known as wet-out) that occurs when two smooth surfaces are separated by less than about 1.5 xcexcm. Optical coupling is particularly serious when one of the surfaces belongs to a lightguide or waveguide that is transmitting light along its length by total internal reflection (TIR). Such coupling serves to provide a path for light to escape from the lightguide in an unwanted manner, causing non-uniform illumination. In a strictly transmissive/reflective mode, the same proximity serves to produce constructive and destructive reflections that make the articles appear to be wet between the surfaces (wet-out) and also appear to have rings at the boundaries, called Newton""s Rings.
Accordingly, in a first aspect, the present invention includes a reflective article comprising a reflective material proximate to a structure. An example of these structures includes, but is not limited to, light guides or hollow light cavities. The reflective article is optionally either a specular reflective material or a diffuse reflective material, and includes a first surface having surface elements configured to reduce or eliminate optical coupling with the structure. Examples of these surface elements include, for example, variable height grooves, pyramids, hemispheres, and coated particles. In a preferred implementation, the reflective article is a diffuse reflector comprising a porous polymeric sheet containing a network of polymeric material having voids therein. The porous polymeric sheet includes a polymer component and a diluent component. The diluent component is miscible with the polymer component at a temperature above the melting point of the polymer component. This porous polymeric sheet preferably includes surface elements configured to reduce or eliminate optical coupling with the structure. These surface elements are formed, for example, by calendering, embossing the polymeric sheet, or selective application of a coating to the polymeric sheet to create the structures.
The porous polymeric sheet can be subject to a mechanical force, such as calendering, to reduce its thickness. The porous polymeric sheet preferably has a reflectivity of greater than 92%, more preferably greater than 95%, and even more preferably greater than 98% at a wavelength of 550 nanometers according to ASTM E 1164-94 measured using a spectrophotometer with an integrating sphere.
A second aspect of the present invention is a diffuse reflective article including a diffuse reflective material proximate to a structure wherein said diffuse reflective material is made of a porous polyolefin sheet comprising an air region and a material region where the material region forms a network of material. One or more surface elements can be added to the polyolefin sheet in order to reduce or eliminate optical coupling with the structure.
A third aspect of the present invention is a diffuse reflective article including a diffuse reflective material proximate to a structure wherein said diffuse reflective material is made of a porous polymeric sheet characterized by a microstructure comprising a network of polymer domains and fibrils interconnecting the domains as shown in FIG. 13.
A fourth aspect of the present invention is a method of improving diffuse reflectivity of light using a diffuse reflective material to cause light energy to reflect off of it, wherein the material includes a porous polymeric sheet having an air region and a material region where the material region forms a network of material containing:
(a) a polymer component, and
(b) a diluent component, said diluent component being miscible
with the polymer component at a temperature above the melting point of the polymer component or a liquid-liquid phase separation temperature of a total solution.
A fifth aspect of the present invention is an optical cavity including a light source in combination with a housing that further contains a diffuse reflector lining a portion of the cavity, the diffuse reflector including a porous polymeric sheet as described above.
A sixth aspect of the present invention is an optical cavity including a light source in combination with a housing that further contains a diffuse reflector lining a portion of the cavity and partially wrapping around the light source so as to direct light from the light source into the optical cavity. The diffuse reflector reflects light from the light source into the optical cavity, and also Reflects light, including recycled light, in the optical cavity toward a viewer.
In certain implementations of the invention, the diffuse reflector incorporates materials imparting resistance to degradation from radiation, including ultraviolet (UV) radiation. In other implementations, the diffuse reflector incorporates a fluorescent compound. In particular, when the light source is ultraviolet light or contains ultraviolet light, the diffuse reflector may incorporate a fluorescent compound that absorbs ultraviolet light and emits visible light.
A seventh aspect of the invention is directed to methods of making reflective articles. The methods including making specular and diffuse reflective articles, and in particular TIPS reflective articles. A specific method of the invention includes making an article comprising a diffuse reflective material configured for attachment to a structure. The method includes providing a polymer component and a diluent component. The diluent component is miscible with the polymer component at a temperature above the melting point of the polymer component or liquid-liquid phase separation temperature of the total solution of polymer and diluent. The polymer and diluent components are combined to form a porous polymeric sheet.
After formation of the porous polymeric sheet, a force is optionally applied to the porous polymeric sheet to reduce its thickness while maintaining a high level of reflectivity. The force may be applied, for example, by calendering the polymeric sheet between calendar rolls. During application of the force, surface elements may be added to the polymeric sheet. Alternatively, the surface elements may be added to the sheet by forming the sheet by embossing, or by depositing a coating on portions of the sheet.
An eighth aspect of the invention is a lamp cavity including a light source, such as a cold cathode fluorescent lamp, in combination with a housing that further contains a diffuse reflector lining a portion of the cavity facing the light source and partially wrapping around the light source. The lamp cavity is preferably integrally formed with a diffuse reflective material proximate to a structure, wherein said diffuse reflective material is made of a porous polymeric material.
The diffuse reflective materials of the present invention have been found to be useful in a variety of structures for light management applications. For example, they have been used as a back reflector in LCD backlight constructions. The diffuse reflective materials of the present invention may also be used to increase the brightness of sign cabinets, light fibers, and light conduits. Such articles containing the diffuse reflective material of the present invention are further aspects of the present invention.
The present invention includes methods of making a diffuse reflective material. One method includes the steps of: (a) melt blending to form a solution comprising about 10 to 90 parts by weight of a polymer component substantially non-absorbing to light to be reflected, and about 10 to 90 parts by weight, based on a total solution content, of a diluent component, said diluent component being miscible with the polymer component at a temperature above the melting temperature of the polymer component, or the liquid-liquid phase separation temperature of the total solution; (b) shaping the solution; (c) phase separating the shaped solution to form phase separated material regions through either (i) crystallization of the polymer component to form a network of polymer domains, or (ii) liquid-liquid phase separation to form networks of a polymer-lean phase; (d) creating regions of air adjacent to the material regions to form the porous article; and optionally (e) applying a mechanical force to the article to reduce its thickness; wherein the article has a reflectivity of greater than 92% at a light wavelength of 550 nanometers as measured according to ASTM E 1164-94 using a spectrophotometer with an integrating sphere.
In certain implementations of the invention, the methods further include forming a plurality of surface elements in the diffuse reflective material. These surface elements are positioned to reduce or eliminate optical coupling with a substrate against which the diffuse reflective material is subsequently placed. The surface elements can be formed, for example, by calendering, embossing the structures into it, or coating a material onto the diffuse reflective material to form the structures.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The drawings and the detailed description that follow more particularly exemplify certain preferred embodiments utilizing the principles disclosed herein.